A machine for effecting printing, e.g., on one of the two faces of a sheet of paper in four colors, using a plurality of electrophotographic printing units is known as shown in JP H07-271107 A.
In such a printer using a plurality of electrophotographic printing units, a plurality of toner image forming units are arranged in a direction of conveyance of a sheet of paper. Each of the toner image forming units comprises a photoconductor drum as a image supporting member, a charging means for charging a surface of the photoconductor drum, an exposure means for exposing the surface of the photoconductor drum to form a latent image on the surface and a developing means for developing the latent image into a toner image, or the like. Toner images formed on the respective photoconductor drums in the toner image forming units are successively transferred onto the sheet of paper by means of a transfer means using a transfer conveyer belt with which the photoconductor drums in the respective toner image forming units are in rotational contact and the sheet of paper is conveyed.
In the conventional printer using a plurality of electrophotographic printers, a sheet of paper is successively conveyed on the transfer conveyer belt along a plurality of the successive toner image forming units. Then, while being conveyed across two adjacent toner image forming units, the sheet of paper is nipped to be applied with printing pressure in two areas at both end portions of the sheet of paper extending in its straight conveyance direction, giving rise to the problem that the accuracy in printing registration may be affected. In a multi-color printer to prevent this problem, the spacing between adjacent toner image forming units has been sized so that a sheet of paper after passing through a nip portion in a toner image forming unit is fed into that in a next toner image forming unit
Also, a printer using an electrophotographic printing unit, development of an image onto a surface of a photoconductor drum and cleaning of the surface of the photoconductor drum having transferred the image can proceed always simultaneously. Thus, a sheet of paper can be printed with an image of a length in its conveyance direction regardless of a circular peripheral length of the photoconductor drum, and even with an image of a length exceeding the circular peripheral length of the photoconductor drum. If, however, sheets of paper are to be printed with such a long image, the spacing between adjacent toner image forming units must be of a size exceeding the maximum image size (length). As a result, the entire multi-color printer in which a plurality of the toner image forming units are interconnected must be correspondingly made longer in total size.
If in such a printer a sheet of paper is to be printed on its both, faces, a sheet of paper having been printed on its one face by one printing unit is reversed in printing face and then passed into a next, identical printing unit disposed adjacent thereto. If the sheet of paper is to be printed on its front and rear faces in four color, the length from the first toner image forming unit to the eighth toner image forming unit must be 7 (seven) times longer than an image size (length). The problem thus arises that the entire multi-color printer is made longer to allow printing an image that is long in the direction of conveyance of the sheet of paper.
Further, since the conventional printers adopt a belt conveyer system, there have been errors in sheet feed due to stretchability of the belt and limitations in accuracy arising from the structural phase of the belt conveyer system, which are more remarkable especially as the printing speed is to be increased. There have thus existed problems in maintaining high precision in printing registration in high speed sheet conveyance.
In view of the foregoing, it is an object of the present invention to provide a sheet-fed duplex and a sheet-fed duplex multi-color printer which is compact, using an electrophotographic printing system and which, if for a long image to be printed on both faces of a sheet of paper, allows producing a printed matter at high efficiency and high precision, and yet wherein in printing front and rear faces of a sheet of paper in succession the printer is able to prevent a printing face of the sheet of paper printed on upstream in its conveyance direction from becoming soiled by toner transferred from a printing face of the sheet of paper printed on downstream in its conveyance direction.